why there is we call thin line between friendship and love

There is a thin line between friendship and love because things will become complicated if either one fell in love with her best friend and the other one wants only friendship. So if you don't want your friendship to break don't cross the border. There is no thin line if both of you are falling in love. The best thing is build first strong foundation in your friendship, when both of you can't go on with life without each other.